The present invention relates to a propeller system, and more particularly to a translational thrust system with a propeller pitch change system which integrates thrust control with a Fly-By-Wire (FBW) system of a high-speed rotary-wing aircraft.
A rotary wing aircraft with a coaxial contra-rotating rotor system is capable of higher speeds compared to conventional single rotor helicopters due in part to the balance of lift between the advancing sides of the main rotor blades on the upper and lower rotor systems. To still further increase airspeed, supplemental translational thrust is provided by a translational thrust system such as a propeller system oriented substantially horizontal and parallel to the aircraft longitudinal axis to provide thrust for high-speed flight.
Variable pitch propellers are common in fixed wing aircraft. These propellers are generally attached directly to the crankshaft of a reciprocating engine, or are mated to a gearbox of a turbine engine. In either case, the speed of the propeller is proportional to the speed of the engine. The propeller is also the only significant load the engine has to drive.
Variable pitch propellers common to fixed wing aircraft are unsuitable for the translational thrust system of a high speed rotary-wing aircraft as a propeller governor controls engine RPM by pitch change of the propeller blades. On a high speed rotary-wing aircraft with a translational thrust system, the pitch of the propeller blades are controlled independent of engine RPM to control translational thrust as the aircraft operates through a range of RPMs based upon flight regime. With the propeller held at a constant pitch, as this invention does for a given trim condition, the torque required at the propeller remains largely constant compared to the main rotor. Conventional variable pitch propeller systems are unsuitable for such independent operation.
Conventional variable pitch propeller systems efficiently convert rotary motion into propulsive thrust, however, conventional variable pitch propeller systems utilize a conventional propeller governor which results in at least two problems for a high-speed rotary-wing aircraft. First, on a conventional fixed wing airplane, the governor is used to control engine RPM by changing pitch of the propeller blades. On high-speed rotary-wing aircraft with a translational thrust system, the requirement is to control thrust by changing the pitch of the blades, with RPM maintained constant by the Electronic Control Unit (ECU) on the engine. The second problem is potential interplay between the ECU and the propeller governor. Consider a disturbance where the RPM of the system is lowered by several percent. The ECU will detect the reduction in RPM and add more fuel as a result. Additionally, the reduction in RPM will cause the flyweights in the propeller governor to swing inward, moving its pilot valve to dump oil out of the hub, and reduce prop pitch. This will reduce the mechanical load of the overall system. With both controllers acting to increase the speed of the system, the response may be greater than either had anticipated, and an overshoot will result. The reaction to an overspeed condition will be similar, where the ECU will reduce the amount of fuel while the governor simultaneously reduces the pitch of the propeller. With two separate controllers trying to maintain RPM, there is the potential for development of a torsional instability.
Accordingly, it is desirable to provide a propeller pitch change system for a translational thrust system of a high-speed rotary-wing aircraft which independently integrates thrust control with a Fly-By-Wire (FBW) system to position the propeller blades with a high degree of pitch angle confidence.